Field of the Invention
The present invention relates to a laser imaging apparatus, and in particular, to a laser imaging apparatus having a velocity control arrangement.
Description of the Prior Art
In an imaging apparatus in which a laser beam scans a photosensitive substrate in a direction perpendicular to the direction of transport of the substrate while the substrate is being transported, it is essential that the transport of the substrate be carried out at a constant speed, i. e., without perturbations, to obtain an image of good quality. Otherwise, imaging artifacts are created on the imaged substrate. Such artifacts include, for example, disturbed images, images of impaired quality, or banding (i.e., irregularities in the density of scanning lines).
U.S. Pat. No. 4,885,759 (Maeda) discloses a laser scanning apparatus which includes a rotatable subscanning drum and at least one nipping roller in pressing contact therewith and rotatable by the drum. The nipping roller(s) has an elastic member thereon which attenuates any vibrations. In one embodiment, a substrate is forced into nipping engagement with the sub,canning drum and two spaced nipping rollers. Imaging of the substrate occurs after the substrate is controlled by a nip of at least one of the nipping rollers and the subscanning drum. The sheet thickness is accommodated by the deformation of the elastic members of the nipping rollers. Therefore, the subscanning drum, the nipping rollers, and the sheet are substantially free of the vibration or torque variation due to the forced engagement. In a second embodiment, the subscanning drum forms a nip with one nipping roller. The substrate may be scanned with a laser beam immediately in front of or to the rear of the nipping roller. The manner in which the speed substrate is controlled when the laser is imaging the substrate prior to entry into the nip is not made clear.
U.S. Pat. No. 4,665,408 (Toriumi et al.) discloses a transport mechanism for transporting a sheet in an imaging apparatus which includes an auxiliary scanning drum, a pair of nip rollers held in contact with the scanning drum, a support plate extending substantially horizontally in a sheet transport path, a push lever pivotally mounted at the end of the support plate adjacent to the nip rollers, a stopper pivotally mounted at the other end of the support plate, and a pair of feed nip rollers. A feed mechanism supplies a sheet to the feed nip rollers which feeds the sheet onto the support plate. The push lever urges the trailing edge of the sheet forward. The stopper contacts the leading edge of the sheet and accurately positions the sheet on the support plate when the stopper and the push lever are retracted from the sheet transport path. After retraction of the stopper the push lever resumes pushing the trailing edge of the sheet until the same is gripped at its leading edge between the rotating auxiliary scanning drum and the rotating nip rollers. The sheet is transported by the nip rollers and the scanning drum. Imaging of the sheet occurs in a space between the nip rollers. There appears to be no teaching that the transporting mechanism can accurately control the transport of the sheet to suitably image the leading edge of the sheet if the laser imaging of the sheet occurs prior to entry into the nip.
U.S. Pat. No. 4,492,455 (Kawai) discloses a laser imaging apparatus which includes a subscanning drum and spaced nipping rolls. The nipping rolls are initially offset from the subscanning drum. The drum continuously rotates at a constant speed. A stopper is disposed downstream of the drum and positioned in the sheet feeding path. A sheet is transported until it abuts against the stopper, and the nip roll or rolls are moved pressing against the sub-scanning drum, thus sandwiching the sheet therebetween. The stopper is retracted from the sheet feeding path whereby the sheet is moved in the sub-scanning direction. The imaging of the film by a laser begins after the film is captured in at least one nip. Since the sheet is captured in the nip before laser imaging begins, the substrate motion is carefully controlled during imaging.
Also, it is believed desirable to image to all edges of the substrate to create a frame-like area (or border) of high density on the substrate surrounding the diagnostic image areas. The high density frame reduces the eyestrain of a diagnostician when reading the diagnostic images on a back-lit lightbox. In the case where a substrate is transported by a single pair of nip rollers, it is difficult to image the leading edge or the trailing edge of the substrate to create a complete border since, at some time during imaging, the substrate is not controlled by nip rollers.
It is also believed desirable to provide an imaging apparatus which is smaller in footprint size than existing apparatuses of this kind. This results in an architecture for the imaging apparatus in which the substrate may be influenced by forces, particularly gravitational forces, during transport of the substrate as the substrate is being imaged. If not accommodated these forces can detrimentally influence the film image quality. So there arises a need in a laser imaging apparatus to control the velocity of a substrate during imaging of the substrate, before the transport of the substrate is controlled by nip rollers. Additionally, the velocity control prior to the nip must be able to introduce the substrate into the nip of the nip rollers during imaging of the substrate without disrupting the substrate motion in any way.